The ability to promote and regulate recombinant gene expression is of importance in research, in the industrial production of cell products, and in the development of effective approaches to gene therapy. Attempts to regulate gene expression in mammalian cells have generally focused on the use of inducible promoters (Brinster, et al., Nature 296:39-42 (1982); Nover, in Heat Shock Response, pp. 167-220, CRC, Fla. (1991)); Klock, et al., Nature 329:734-736 (1987)) or on the use of prokaryotic regulatory elements (see e.g., Labow, et al., Mol. Cell. Biol. 10:3343-3356 (1990); Brown, et al., Cell 49:603-612 (1987); Kim, et al., J. Virol. 69:2565-2573 (1995); Hennighausen, et al., J. Cell. Biochem. 59:463-472 (1995); Deuschle, et al., Mol. Cell. Biol. 15:1907-1914 (1995)).
One particularly effective system for regulating gene expression in mammalian cells uses a tetracycline-inducible transcription switch (U.S. Pat. Nos. 6,444,871; 6,251,640; 5,972,650; Yao, et al., Hum. Gene Ther. 9:1939-1950 (1998)). Gene expression is suppressed in this system by the binding of the tetracycline repressor, tetR, to a tetracycline operator (tetO) sequence that has been inserted downstream of the TATA element (TATATAA) in an hCMV major immediate-early promoter. In order to turn on expression of the gene sequence, tetracycline is introduced into the system. This enters into cells, binds to the repressor protein and causes it to dissociate from the operator. This system has been used in commercially available plasmids (T-Rex System™, Invitrogen™, Carlsbad, Calif.), in HSV vectors designed to deliver therapeutic genes to cells (US 20050266564) and in oncolytic viruses (US 20080008686).
A problem that has limited the use of the systems described above, particularly in the area of gene therapy, is that the vectors used to deliver recombinant DNA to cells often have a very limited capacity. For example, Adeno-associated viral vectors are among the most promising for gene therapy but can only accommodate a few kilobases of DNA (Ghosh, et al., Genet. Eng. Rev. 24:165-178 (2007)). Ways to more efficiently use the space available in such vectors should expand their utility.